1. Field of the Invention
The present invention relates to a desynchronizer for high speed telecommunication signals. The invention more particularly relates to an apparatus for generating an ungapped E4 (DS4NA) signal from the gapped data component of an STM-1 (STS-3C) payload signal.
2. State of the Art
The telecommunications network servicing the Unites States and the rest of the world is presently evolving from analog transmission to digital transmission with ever-increasing bandwidth requirements. Fiber optic cable has proved to be a valuable tool of such evolution, replacing copper cable in nearly every application from large trunks to subscriber distribution plants. Fiber optic cable is capable of carrying much more information than copper with lower attenuation.
While fiber optic cable represents the future in telecommunications, presently there remains an entire telecommunications network comprised of various cable types, served by equipment of different vintages, and run according to various coexisting transmission standards. While older standards, cables, and equipment will be eventually phased out, for the time being it is necessary that all the old and new standards, equipment, and transmission lines be as compatible as possible. For example, in a wire plant, every signal should be connectible to every other signal.- To achieve this, it is not enough to simply multiplex signals from lower to higher orders and vice-versa. In addition to a mux/demux function, signal format conversion operations must be performed before connectibility can be achieved. For instance, a DS-4NA signal cannot simply be connected to an STS-3C signal as these signals are at different rates (155.52 Mb/sec.+-.4.6 ppm for the STS-3C signal, and 139.264 Mbit/sec.+-.20 ppm for the DS-4NA signal) and use different multiplexing formats. Thus, a conversion from a DS-4NA or E4 signal to an STS-3C or STM-1 signal requires the addition of overhead bytes, stuff, control information, etc. which are accommodated in an increased data rate. Likewise, in recovering the DS-4NA or E4 signal from the STS-3C signal in which it is carried, the overhead bytes, stuff, control information, etc. must be stripped out of the STS-3C signal as seen in the prior art FIG. 1, thereby producing gaps in the clock of the extracted DS-4NA signal from which an ungapped slower DS-4NA signal must be regenerated.
As seen in FIG. 1, for each row of two hundred seventy bytes of an STS-3C signal, there are nine bytes of transport overhead and one byte of path overhead which are typically removed from the data signal. Of the remaining two hundred sixty bytes, and as seen in FIG. 2, there are thirteen bytes of fixed stuff (RRRRRRRR) denoted "Y" which must be removed, five bytes denoted "X" which includes a stuff control bit (C), five bits of fixed stuff (R), and two bits of overhead communications which must be removed, and one byte denoted "Z" which includes five payload bits (I), one stuff opportunity bit (S), and one fixed stuff bit (R). In the "Z" byte, either one or two bits are removed depending upon whether stuff opportunity bit S of the particular signal contains data or stuff. Knowledge of whether bit S is a stuff or a data signal is obtained from the stuff control signals C which are part of the "X" bytes. Details of the STS-3C frame format and the means used to remove the overhead, stuff, and control information from the STS-3C signal are not particularly relevant to the instant invention, but may be seen with reference to prior art documents: Bellcore TR-NWT-000253; ANSI - T1.105-1991; and ITU (formerly CCITT) Recommendation G.709. What is relevant, is that the data signal received from whatever is removing the overhead, stuff and control information of the STS-3C signal is a severely gapped data signal with a one thousand, nine hundred, thirty-four or thirty-five data bits per row at a clock rate of 155.52 Mb/sec .+-.4.6 ppm, and an average rate of 139.264 Mbit/sec.+-.20 ppm. This gapped STS-3C data payload signal must be transformed into an ungapped DS-4NA signal at the 139.264 Mbit/sec.+-.20 ppm rate.
It should be noted that in the case of a negative pointer movement, the three H3 bytes of the transport overhead of the STS-3C signal are to be used for data (i.e., a data destuff). In this case, extra data bits will be placed into the frame. The number of extra data bits placed into the frame depends upon the phase of telecommunications signal. For example, if all three bytes are data bytes, twenty-four bits of data will be added to the frame. However, if one of the bytes being placed into the H3 byte locations is an X or Y byte, only sixteen extra bits of data are added to the frame. Likewise, if one of the bytes being placed into the H3 byte locations is a "Z" byte, twenty-two or twenty-three extra bits are added. Conversely, in the case of a positive pointer movement, the three bytes after the H3 TOH bytes are used as stuff bytes, and depending upon the phase of the signal the frame will contain sixteen, twenty-two, twenty-three, or twenty-four fewer bits of data.
While the apparatus and techniques disclosed in the related patents hereto provide excellent tools for desynchronizing telecommunication signals, it will be appreciated that those apparatus and techniques primarily involve signals which are of the DS-3 type or lower speed. While generally applicable, at the higher frequencies of the STS-3C and E4 signals they do not work as well as might be desired because they require the use of an extremely high speed bit serial data path through the device.